Oct 22, 2018 - Morocco; NWO, Netherlands; RCN, Norway; MNiSW and NCN, .... and P. Skands, PYTHIA 6.4 physics and manual, JHEP 05 (2006) 026,.
EUROPEAN ORGANISATION FOR NUCLEAR RESEARCH (CERN)
arXiv:1810.08424v1 [hep-ex] 19 Oct 2018
Submitted to: EPJC
CERN-EP-2018-259 22nd October 2018
Measurements √ of W and Z boson production in p p collisions at s = 5.02 TeV with the ATLAS detector The ATLAS Collaboration
Measurements of fiducial integrated and differential cross sections for inclusive W + , W − and Z boson production are reported. They are based on 25.0 ± 0.5 pb−1 of pp collision data √ at s = 5.02 TeV collected with the ATLAS detector at the CERN Large Hadron Collider. Electron and muon decay channels are analysed, and the combined W + , W − and Z integrated cross sections are found to be σW + = 2266 ± 9 (stat) ± 29 (syst) ± 43 (lumi) pb, σW − = 1401±7 (stat)±18 (syst)±27 (lumi) pb, and σZ = 374.5±3.4 (stat)±3.6 (syst)±7.0 (lumi) pb, in good agreement with next-to-next-to-leading-order QCD cross-section calculations. These measurements serve as references for Pb+Pb interactions at the LHC at this nucleon–nucleon centre-of-mass energy.
© 2018 CERN for the benefit of the ATLAS Collaboration. Reproduction of this article or parts of it is allowed as specified in the CC-BY-4.0 license.
1 Introduction Measurements of W ± and Z boson1 production at hadron colliders provide a benchmark for the understanding of quantum chromodynamics (QCD) and electroweak (EW) processes. Predictions for the differential and fiducial cross sections are available up to next-to-next-to-leading-order (NNLO) accuracy in QCD and include EW corrections at next-to-leading-order (NLO) accuracy [1–3]. The rapidity distribution of EW boson production is sensitive to the underlying QCD dynamics and, in particular, to the parton distribution functions (PDFs) which define the initial kinematics of the hard process. Therefore, measurements of weak-boson production offer an excellent opportunity to test models of parton dynamics. The ATLAS, CMS and LHCb collaborations have measured W ± and Z boson production in proton– √ proton (pp) collisions at centre-of-mass energies of s = 7, 8 and 13 TeV [4–7]. These measurements provide precision tests of the QCD theory and PDFs, which can be complemented with measurements at √ the additional centre-of-mass energy s = 5.02 TeV. This paper describes measurements of the production cross sections times leptonic branching ratios for the inclusive W + → ` + ν, W − → ` − ν and Z → ` + ` − (` = e, µ) processes. Integrated and differential cross sections are measured in a fiducial phase space defined by detector acceptance and lepton kinematics. For W ± bosons the decay lepton charge asymmetry is also determined. All measurements are performed √ with pp collision data corresponding to an integrated luminosity of 25.0 pb−1 , collected at s = 5.02 TeV with the ATLAS detector. The data were recorded during the autumn of 2015. The peak instantaneous luminosity delivered by the LHC was L = 3.8 × 1032 cm−2 s−1 and the mean number of pp interactions per bunch crossing (hard scattering and pile-up events) was 1.5. Therefore, this dataset is characterised by a relatively low pile-up contribution as compared to the measurements of weak-boson production performed at higher centre-of-mass energies by ATLAS. In addition, the measurement of W ± and Z boson production in pp collisions at the centre-of-mass energy √ s = 5.02 TeV is an important reference for weak-boson production in heavy-ion collisions. The LHC has provided both proton–lead (p+Pb) and lead–lead (Pb+Pb) collisions at the centre-of-mass energy per √ nucleon pair sNN = 5.02 TeV. Published results from the ATLAS and CMS collaborations are currently √ available for W ± and Z boson production [8–11] in Pb+Pb collisions at sNN = 2.76 TeV and Z boson √ production [12, 13] in the p+Pb system at sNN = 5.02 TeV.
2 The ATLAS detector The ATLAS experiment [14] is a multipurpose particle detector with a forward–backward symmetric cylindrical geometry.2 It consists of an inner tracking detector surrounded by a thin superconducting solenoid, electromagnetic and hadronic calorimeters, and a muon spectrometer incorporating three large superconducting air-core toroid magnets with eight coils each. The inner-detector system (ID) is immersed in a 2 T axial magnetic field and provides charged-particle tracking in the pseudorapidity range |η| < 2.5. At small radii, a high-granularity silicon pixel detector 1 2
Throughout this paper, Z/γ ∗ boson production is referred to as Z boson production. ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the centre of the detector and the z-axis along the beam pipe. The x-axis points from the IP to the centre of the LHC ring, and the y-axis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the z-axis. The pseudorapidity is defined in terms of the polar angle θ as η = − ln tan(θ/2).
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covers the interaction region and typically provides four measurements per track. It is followed by the silicon microstrip tracker, which usually provides eight measurement points per track. These silicon detectors are complemented by a gas-filled straw-tube transition radiation tracker, which enables track reconstruction up to |η| = 2.0. The transition radiation tracker also provides electron identification information based on the fraction of hits (out of ∼ 35 in total) with an energy deposit above a threshold indicative of transition radiation. The calorimeter system covers the pseudorapidity range |η| < 4.9. Within the region |η| < 3.2, electromagnetic (EM) calorimetry is provided by high-granularity lead/liquid-argon (LAr) calorimeters, with an additional thin LAr presampler covering |η| < 1.8 to correct for upstream energy-loss fluctuations. The EM calorimeter is divided into a barrel section covering |η| < 1.475 and two endcap sections covering 1.375 < |η| < 3.2. For |η| < 2.5 it is divided into three layers in depth, which are finely segmented in η and φ. Hadronic calorimetry is provided by a steel/scintillator-tile calorimeter, segmented into three barrel structures within |η| < 1.7 and two copper/LAr hadronic endcap calorimeters covering 1.5 < |η| < 3.2. The solid-angle coverage is completed with forward copper/LAr and tungsten/LAr calorimeter modules in 3.1 < |η| < 4.9, optimised for electromagnetic and hadronic measurements, respectively. The muon spectrometer (MS) comprises separate trigger and high-precision tracking chambers measuring the deflection of muons in the magnetic field generated by the toroid magnets. The precision chamber system covers the region |η| < 2.7 with three layers of monitored drift tubes, complemented by cathode strip chambers in the forward region. The muon trigger system covers the range |η| < 2.4 with resistive plate chambers in the barrel, and thin gap chambers in the endcap regions. In 2015, the ATLAS detector had a two-level trigger system [15]. The level-1 trigger is implemented in hardware and uses a subset of detector information to reduce the event rate to a value of at most 75 kHz. This is followed by a software-based high-level trigger which reduces the event rate to about 1 kHz.
3 Simulated event samples Samples of Monte Carlo (MC) simulated events are used to evaluate the selection efficiency for signal events and the contribution of several background processes to the analysed dataset. All of the samples are processed with the Geant4-based simulation [16, 17] of the ATLAS detector. Dedicated efficiency and calibration studies with data are used to derive correction factors to account for residual differences between experiment and simulation, as is subsequently described. The processes of interest, specifically events containing W ± or Z bosons, were generated with the PowhegBox v2 MC program [18] interfaced to the Pythia 8.186 parton shower model [19]. The CT10 PDF set [20] was used in the matrix element, while the CTEQ6L1 PDF set [21] was used with the AZNLO [22] set of generator-parameter values (tune) for the modelling of non-perturbative effects in the initial-state parton shower. The Photos++ v3.52 program [23] was used for QED radiation from electroweak vertices and charged leptons. Samples of top-quark pair (t t¯) and single-top-quark production were generated with the Powheg-Box v2 generator, which uses NLO matrix element calculations together with the CT10f4 PDF set [24]. Top-quark spin correlations were preserved for all top-quark processes. The parton shower, fragmentation, and underlying event were simulated using Pythia 6.428 [25] with the CTEQ6L1 PDF set and the corresponding Perugia 2012 tune (P2012) [26]. The top-quark mass was set to 172.5 GeV. The EvtGen v1.2.0 program [27] was used to model bottom and charm hadron decays for all versions of Pythia. Diboson processes were simulated using the Sherpa v2.1.1 generator [28]. They were calculated for up to
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one (Z Z) or zero (WW, W Z) additional partons at NLO QCD accuracy and up to three additional partons at LO. In addition, the Sherpa diboson sample cross section is scaled to account for the cross section change when the G µ scheme [29] is used instead of the native one for the EW parameters, resulting in an effective value of α ≈ 1/132. Multiple overlaid pp collisions were simulated with the soft QCD processes of Pythia v8.186 using the A2 tune [30] and the MSTW2008LO PDF set [31]. For the comparison with data in differential distributions and the evaluation of single-boson EW backgrounds for the cross-section calculations, the single-boson simulations are normalised to the results of NNLO QCD calculations [2, 3], with uncertainties of 3%. The simulations of all other processes are normalised to the predictions of NLO QCD calculations, with uncertainties of 10% for the diboson and top-quark processes.
4 Object definitions and event selection This section describes the reconstruction of electrons, muons and hadronic recoil objects, and the selection of W and Z bosons. Candidate events are required to have at least one primary vertex reconstructed from at least three tracks with pT > 400 MeV and to pass a trigger selection, which requires a single electron or muon candidate with a pT threshold of 15 GeV or 14 GeV, respectively. In addition, a loose likelihood-based identification requirement [32, 33] is applied in the electron trigger. Electron candidates are required to have pT > 20 (25) GeV in the Z (W) boson analysis and |η| < 2.47. Candidates within the transition region between barrel and endcap calorimeters (1.37 < |η| < 1.52) are rejected. In addition, medium likelihood-based identification and tight isolation requirements are applied [32, 33]. Muon candidates must satisfy pT > 20 (25) GeV in the Z (W) boson analysis and |η| < 2.4 and pass the requirements of medium identification and tight isolation [34]; both criteria were optimised for 2015 analysis conditions. Additional requirements are imposed on the significance of the transverse impact parameter, d0 , such that |d0 |/σd0 < 5 (3) for electron (muon) candidates. To ensure that lepton candidates originate from the primary vertex, a requirement is also placed on the longitudinal impact parameter, z0 , multiplied by the sine of the track polar angle, θ, such that the absolute value is smaller than 0.5 mm. Events with Z boson candidates are selected by requiring exactly two opposite-charge electrons or muons, at least one of which is matched to a lepton selected at trigger level. The dilepton invariant mass must satisfy the fiducial requirement 66 < m`` < 116 GeV. Events with W boson candidates are selected by requiring exactly one electron or muon that is matched to a lepton selected at trigger level. In events where a W boson is produced and decays leptonically, the full event kinematic reconstruction is not possible due to the presence of an (anti-)neutrino that escapes direct detection. A measure of the missing transverse momentum corresponding to the neutrino, pνT , can be inferred from information about the hadronic system recoiling against the W boson. The hadronic recoil is the vector sum of all calorimeter energy clusters excluding the deposits from the decay muon or electron, and is further described below. The transverse projection of the recoil onto the r–φ plane, u®T , is used together with the decay lepton transverse momentum p®T` for the calculation of the missing transverse momentum vector, E®Tmiss = − u®T + p®T` ,
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whose magnitude is denoted ETmiss . The transverse mass of the lepton-ETmiss system is defined as r mT = 2p`T ETmiss 1 − cos ∆φ`,E miss where ∆φ`,E miss is the azimuthal angle between p®T` and E®Tmiss . T
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Fiducial selections ETmiss > 25 GeV and mT > 40 GeV, optimised for the multi-jet background reduction, are required for events with W boson candidates. The general structure of the algorithm used for hadronic recoil reconstruction is introduced in Ref. [35], where three-dimensional topological clusters [36] calibrated at the hadronic scale are used as inputs to the algorithm. In this measurement, the hadronic recoil is reconstructed using particle flow objects [37] as inputs. The ATLAS particle flow algorithm provides an improved ETmiss resolution compared to the algorithm using only topological clusters, and makes the measurement less sensitive to pile-up by separating the charged-hadron contribution from the neutral hadronic activity [37]. The charged activity is measured by the ID and the related tracks from charged hadrons can be matched to a vertex. From all charged hadrons, only calorimetric clusters associated with a track originating from the reconstructed primary vertex are retained as input to the hadronic recoil algorithm. The neutral hadronic activity is represented by clusters without an associated track, and is also used in the recoil algorithm.
5 Detector performance corrections 5.1 Lepton calibration and efficiency The electron energy calibration is primarily obtained from the simulation by employing multivariate techniques [38]. The signal Z → ee MC simulation is used for deriving the data energy scale calibration and resolution corrections for the simulation. The energy resolution is corrected with additional factors no larger than about 1% in the barrel and up to 2% in the endcap region of the detector in order to account for a slightly worse resolution observed in the data. The energy scale is corrected by applying a per-electron energy scale factor to the data derived from a comparison of the electron-pair invariant mass between the simulation and the data. This procedure was found to be sensitive to the pile-up distribution in data due to different settings used for the signal readout from the EM calorimeters [39]. Therefore, a special set of scale correction factors was derived for this dataset. Measurements of muon momenta can be biased by the detector alignment and resolution, distortions of the magnetic field or imprecise estimates of the amount of passive material in the detector. Corrections of the muon momentum scale and resolution, which are applied to the simulation, are derived as a function of the muon η and φ using Z → µ+ µ− events [34]. The correction factors are chosen such that they minimise the χ2 between the muon-pair invariant mass distributions in data and simulation. Electron candidates used for the analysis are required to satisfy selection criteria related to reconstruction, identification, isolation and trigger. For each of these requirements, the efficiency of the selection is measured in data with the tag-and-probe method in Z → e+ e− events, as described in Ref. [33], and compared with the simulation. Data-to-simulation ratios of efficiencies are used as scale factors to correct the simulation for the observed differences. Measurements are performed as a function of the electron pT and η for electrons selected in the analysis. All uncertainties related to efficiency are classified as either correlated or uncorrelated, and are propagated accordingly to the final measurement uncertainty. The electron reconstruction efficiency is in the range 95–99% both in the data and simulation and is typically measured with a precision of 2%. The data-to-simulation ratio is up to 2% (5%) different
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from unity in the barrel (endcap) calorimeter and is measured typically with 2% precision for pT in the range ∼30–50 GeV and 5% for pT > 60 GeV. The efficiency of an electron to further pass the medium identification definition varies from 85% to 95% and is measured with 2% precision. This efficiency differs from the efficiency measured in the MC simulation by up to 5%. The isolation efficiency is measured with a precision of 5% and agrees with the simulated value within 2%. Data-to-simulation correction factors for identification and isolation efficiencies are measured with a precision of 2–6%. Finally, the trigger efficiency data-to-simulation ratio is found to deviate from unity by 0.5–3% and is measured with a precision of up to 2%. Various selection requirements related to muon trigger, reconstruction, identification and isolation are imposed on muon candidates used in the analysis. The efficiency of the selection criteria is measured in data with the tag-and-probe method in Z → µ+ µ− events [15, 34] and compared with the simulation. Ratios of the efficiencies determined in data and simulation are applied as scale factors to correct the simulated events. For muons with pT > 20 GeV, the correction factors measured as a function of muon pT have typically an uncertainty of 1–2% and do not deviate from a constant value by more than 3%. Therefore, the pT dependence of the scale factors is neglected, and they are evaluated only as a function of muon η. The muon trigger efficiency in the endcap region of the detector (1.05 < |η| < 2.4) is measured to be around 90%, and the values obtained in data and simulation agree well. However, in the barrel region (|η| < 1.05) the trigger efficiency determined in the simulation varies from 70% to 85%, while the efficiency measured in data is lower by 5–15%, which results in sizeable scale factors. The combined reconstruction and identification efficiency for medium-quality muons typically exceeds 99% in both the data and simulation with good agreement between the two measurements. The efficiency of the isolation selection is found to be 97–98% in the MC simulation and it differs from the efficiency measured in the data by about 2% in the most central (|η| < 0.6) and most forward detector regions (1.74 < |η| < 2.4). All measurements of lepton efficiency corrections are limited in their precision by the number of Z → ` + ` − √ candidates available in the s = 5.02 TeV dataset. Figure 1 summarises the reconstruction, identification, isolation and trigger efficiencies for electron and muon candidates obtained from the tag-and-probe method. Figure 2 shows the invariant mass distribution of the dilepton system for electron and muon candidates from Z → ` + ` − boson decays after applying scale factors to the MC simulation. The data points are compared with simulation including Z boson signal and background components. The electron candidates in the data, shown on the left panel, are calibrated using calorimeter settings and calibration correction factors optimised for low-pile-up conditions. Good agreement between the data and the simulation is found for both channels.
5.2 Recoil calibration In events with W or Z boson production, the hadronic recoil gives a measure of the boson transverse momentum. The calibration of the recoil is performed using dilepton events from decays of Z bosons √ produced in pp collisions at s = 5.02 TeV, as information about the Z boson transverse momentum can be obtained with high precision from the measurements of lepton momenta and compared with the measurement from hadronic recoil. The recoil resolution is studied using u⊥ , the projection of u®T onto the axis – in the transverse plane – perpendicular to the Z boson p®T . The resolution is given by the standard
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Figure 2: Detector-level invariant mass distribution of (a) dielectron and (b) dimuon pairs from Z boson decays together with EW background contributions. Background contributions are too small to be visible on a linear scale. Only the statistical uncertainties of the data are shown.
deviation of the u⊥ distribution, σu⊥ . The transverse momentum scale response of the recoil can be studied using the bias defined as u k + pTZ , where u k is the projection of u®T onto the axis defined by p®TZ , and is quantified via the average of the bias distribution. Differences between the responses in data and simulation are less than ∼2 GeV, while up to ∼20% differences in the resolution are observed. Following the procedure described in Ref. [35], in situ corrections to the resolution and the scale of uT are obtained in Z events and are applied to the W boson event candidates, as a function of pW T . The corrections
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applied to the simulation are obtained as a function of pTZ : uW,corr k
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Equation (1) describes corrections applied to the recoil response in simulation. It includes a shift which brings the average value of u k in the simulation closer to the one in data, taking into account differences in the bias. In addition, it corrects the response distribution for resolution differences (last term in the equation). The resolution correction is directly described by Eq. (2) where it is applied to the u⊥ distribution in the simulation. The impact of the calibration on the scale and resolution in events where a Z boson decays to a dimuon pair is shown in Figure 3. The distributions are shown for the simulation before and after applying the corrections and for data. Agreement of the distributions from simulation with data distributions is improved after applying the calibration, and residual differences are covered by the systematic uncertainties described in Section 8.
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Figure 3: Distributions of (a) u k + pTZ and (b) u⊥ in data and Z → µ+ µ− MC simulation before (squares) and after (circles) recoil calibration. The shaded band in the ratio panels represents the statistical uncertainty of the data sample, while the error bars represent the systematic uncertainty associated with the calibration procedure.
6 Background determination 6.1 W channels The reported cross-section measurements correspond to inclusive Drell–Yan production of single vector bosons which decay leptonically. Background processes that contribute to the W ± boson production
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measurement are EW processes producing W ± → τ ± ν, Z → ` + ` − , Z → τ + τ − decays, EW diboson (WW, W Z, Z Z) production, as well as top-quark production and multi-jet processes. The multi-jet background includes various processes such as semileptonic decays of heavy-flavour hadrons or in-flight decays of kaons and pions for the muon channel, as well as photon conversions or misidentified hadrons for the electron channel. The background contributions from EW and top-quark production are evaluated using simulated event samples, while the multi-jet contribution is estimated with a data-driven method similar to the one described in Ref. [5]. Although multi-jet background events are well rejected by the lepton isolation requirements, their contribution to the signal region is still sizeable because of the very large production cross sections for multi-jet processes. This contribution is estimated from template fits to data in kinematic distributions: lepton pT , ETmiss and mT . The fits are performed in a phase-space region defined by the full event selection with a looser lepton pT requirement of pT > 20 GeV and with the requirements on ETmiss and mT removed. An additional requirement on the transverse component of the hadronic recoil, uT < 30 GeV, is placed to ensure better agreement of the event kinematics between the fit region and the signal region. Template distributions for signal, EW and top-quark background processes are constructed by applying the fit-region selection to samples of simulated events. Templates enriched in contributions from multi-jet processes are built using events in data with non-isolated leptons selected by inverting the isolation requirement described in Section 4. The normalisation factors of template distributions for signal, EW and top-quark backgrounds, as well as the multi-jet background, are extracted from a fit to the data. The fits are repeated with multi-jet background templates constructed from different intervals in a track-based (muon channel) or calorimeter-based (electron channel) isolation variable. Finally, a linear extrapolation to the signal region is performed as a function of the selected isolation variable, accounting also for the difference in kinematic selections between the fit region and the signal region. Examples of post-fit template ETmiss distributions, which are used to extract multi-jet yields in the electron and muon channels, are presented in Figure 4.
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Following this procedure, multi-jet background processes are estimated to contribute around 0.9% of the W + → e+ ν sample and 1.4% of the W − → e− ν sample, while in the muon channel they represent around 0.1% of the W + → µ+ ν sample and 0.2% of the W − → µ− ν sample.
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Figure 4: Distributions of ETmiss used to extract multi-jet yields in the (a) electron and (b) muon channels after performing the template fits. Only the statistical uncertainties of the data are shown.
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The largest background contributions to the decay modes studied come from the production of single EW bosons decaying via other decay channels. The Z → e+ e− background represents 0.1% of the W + → e+ ν sample and 0.2% of the W − → e− ν sample, while the Z → µ+ µ− background amounts to 2.8% and 3.8% in the W + → µ+ ν and W − → µ− ν samples, respectively. The W ± → τ ± ν background contributes around 1.8% to the samples selected in both channels and the Z → τ + τ − background contributes approximately 0.1%. Contributions from top-quark production (t t¯ and single top quarks) are estimated to be at the level of 0.1–0.2% in both channels. Similarly, diboson processes represent approximately 0.1% of the selected event samples.
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Figures 5 and 6 show detector-level lepton pseudorapidity distributions for positive and negative electron and muon candidates from W boson decays. Good agreement is found between the data and the sum of signal and background contributions.
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6.2 Z channels Background contributions to the Z boson sample are expected from Z → τ + τ − , diboson and W boson decay processes, top-quark pair production, and the multi-jet background. The EW and top-quark contributions are evaluated from dedicated simulation samples, whereas the upper limit on the amount of the multi-jet background is estimated. Diboson background contributes 0.08% in the muon channel and 0.14% in the electron channel. The Z → τ + τ − background is found to be at the level of 0.07% in both decay channels. The top-quark background is at the level of 0.06% in the electron channel and 0.08% in the muon channel. The W boson background is found to be below 0.01% in both channels.
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The contribution of the multi-jet background in the muon channel is estimated from samples that simulate bb¯ and c c¯ production. The study yields an estimate at the level of
